Belgium-based research center imec has announced an improvement of its 4cm2 perovskite/silicon tandem photovoltaic module achieving a power conversion efficiency of 23.9%. Reaching this level, imec claims it is the first to achieve a module-on-cell stack that outperforms the standalone silicon solar cell.

According to imec, perovskite solar cells have many desirable properties, as they can achieve a high power conversion efficiency, are inexpensive to produce, and have a high absorption efficiency in sunlight. The material can be engineered to result in various optical and electronic properties. Additionally, perovskite solar cells or modules may also be used to boost standard silicon (Si) solar technology when engineered to absorb a spectral range that is complementary to the optical range of silicon cells. By stacking the perovskite solar cells or modules on top of Si solar cells, imec continues, power conversion efficiencies above 30% can potentially be achieved, thereby surpassing the efficiencies of the best single junction Si solar cells.

In 2016, imec presented for the first time a semi-transparent perovskite module, developed in collaboration with its Solliance initiative partners, stacked on top of an interdigitated back-contact (IBC) crystalline silicon solar cell in a four-terminal tandem configuration, thereby achieving an overall power conversion efficiency of 20.2% on an aperture area of 4cm2. imec now reports a significant improvement of this technology resulting in a record high power conversion efficiency of 23.9% for the module-on-cell stack of this size.

“Two innovations are key to this achievement,” explains Tom Aernouts, group leader for thin-film photovoltaics at imec and perovskite PV program manager at Solliance. “First, a different perovskite material (CsFAPbIBr) was used, largely improving the stability and conversion efficiency of the 4cm² semi-transparent perovskite module to 15.3 percent. Second, the architecture of the stack was optimized for minimal optical losses by adding an anti-reflection texture on top of the module and a refractive index matching liquid between the perovskite module and the Si solar cell.”

The perovskite/silicon four-terminal tandem was realized with matched aperture areas as large as 4cm2 for the perovskite module and the Si solar cell.

“Having matched areas of this size makes the fabrication technology more attractive to the solar cell industry,” says Aernouts. “For reference, we have also fabricated a stack of a small perovskite cell (0.13cm2) on top of an IBC c-Si cell (4cm2). In this configuration, the power conversion efficiency of the small semi-transparent perovskite cell is 16.7 percent, outperforming the larger 4cm2 perovskite module due to better perovskite layer properties. Although less attractive from an industrial point of view, the overall power conversion efficiency of this cell-on-cell stack is as high as 25.3 percent.”